Conventionally, in surveillance camera systems and the like, a face-image detecting technology that automatically recognizes whether a human face is present in an image taken by a surveillance camera is known. A subspace method and an Integral Image method are examples of the face-image detecting technology.
As an example of the face-image detecting technology using the subspace method, a technology has been developed that can detect a face image by recognizing face parts such as the eyes and the nose in an image by using a subspace, integrating recognition results of the face parts so as to absorb individual differences in the position of each face part, and calculating a reference point of a face (refer to Patent Document 1).
As an example of the face-image detecting technology using the Integral Image method, a technology has been developed that can detect a face image by setting a plurality of rectangular areas in an image, and comparing total values of feature amounts of all the pixels included in each rectangular area (refer to Patent Document 2 and Non-Patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-open No. 2004-54442    [Patent Document 2] Japanese Patent Application Laid-open No. 2004-362468    [Non-Patent Document 1] Paul Viola and Michael Jones, “Rapid Object Detection using a Boosted Cascade of Simple Features”, In Proceedings of IEEE Computer Society Conference on Computer Vision and Pattern Recognition, Vol. 1, pp. 511-518, December 2001